The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image")with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellent ink-receptive areas on a water-receptive ink-repellent background.
The DTR-image can be formed in the image receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image receiving layer in water-permeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
Two main types of mono-sheet DTR materials that are distinct because of their different layer arrangement and processing are known. The first type of mono-sheet DTR material comprises on a support, generally paper or a resin support such as polyester, in the order given a silver halide emulsion layer and an image receiving layer containing physical development nuclei as a surface layer. After information-wise exposure and development according to the DTR process a silver image is formed in the surface layer. Since the underlying layers are hydrophilic in nature and the silver image formed on the surface is hydrophobic or can be rendered hydrophobic the thus obtained plate can be used without further processing. These type of printing plates have a low printing endurance typically around 10000 copies.
On the other hand mono-sheet DTR materials are known that comprise a hydrophilic support provided with an image receiving layer containing physical development nuclei and on top thereof a silver halide emulsion layer. After information-wise exposure and development according to the DTR-process a silver image is formed in The image receiving layer. In order to obtain a lithographic printing plate it will then be necessary to remove the now useless silver halide emulsion layer to expose the silver image formed in the image receiving layer. Said removal is generally carried out by rinsing the element with cold or warm water. This type of printing plate is disclosed in e.g. EP-A-278766, EP-A-483415 and EP-A-410500.
As a preferred support for the latter type of printing plates a toughened and anodized aluminum foil is used and high printing endurances can in principle be obtained. Such type of supports are well known for preparing printing plates using an imaging element having as a light sensitive coating photopolymers (hereinafter called PS-plates) instead of silver halide and are disclosed in e.g. DE-3717757, EP-A-167751, DE-3036174, U.S. Pat. No. 4336113, U.S. Pat. No. 4374710, U.S. Pat. No. 3980539, U.S. Pat. No. 3072546, U.S. Pat. No. 3073765, U.S. Pat. No. 3085950, U.S. Pat. No. 3935080 and U.S. Pat. No. 4052275.
However the requirements imposed on the aluminum foils for use as supports for PS-plates are different from the requirements imposed on the aluminum foils for use in the silver salt diffusion transfer process. Indeed, commonly employed aluminum foils as supports for PS-plates are not suited for preparing printing plates according to the silver salt diffusion transfer process.
In order to obtain printing plates according to the DTR process having good printing properties i.e. good ink acceptance in the image areas, no ink acceptance in the non-image areas called staining or toning and high printing endurances it is required that the adhesion of the image receiving layer containing the physical development nuclei to the aluminum foil is firm. When the adhesion of the image receiving layer to the aluminum foil is poor a certain amount of silver image deposited in the image receiving layer will be removed together with the silver halide emulsion layer during rinsing of the imaging element so that the actual yield or amount of silver deposited in the image receiving layer will be low and as a consequence the printing endurance may be low. Furthermore if the adhesion of the image receiving layer to the aluminum foil is low the silver image will also be worn away more rapidly during printing.
EP-A 567178 discloses a method for preparing an aluminum foil having an improved adhesion for an image receiving layer containing physical development nuclei comprising the steps of roughening and anodizing an aluminum foil and posttreating said toughened and anodized aluminum foil with an aqueous bicarbonate containing solution. However, the result could still use some improvement.